1. Field of the Invention
The present invention relates to a charged-particle beam accelerator which emits a high-energy particle beam produced by accelerating along a circulating orbit a low-energy beam introduced from an ion source, as well as a particle beam radiation therapy system employing such a charged-particle beam accelerator and a method of operating the particle beam radiation therapy system.
2. Description of the Background Art
Conventionally, charged-particle beams produced by circular accelerators like a synchrotron are used for physical experiments and medical applications. The circular accelerator generates a particle beam by accelerating charged particles along a circulating orbit. The charged-particle beam is taken out of the circulating orbit and delivered to a location where the beam is used for a physical experiment or medical treatment through a beam transport line. In one beam extraction technique employed in the circular accelerator, a high-frequency electric field is applied to a circulating beam to increase the amplitude of betatron oscillation up to a point where the betatron oscillation exceeds a stability limit and the charged-particle beam is extracted to the exterior, in which beam extraction is started and stopped by turning on and off the high-frequency electric field.
One example of this kind of approach is identified in Japanese Examined Patent Publication No. 2596292. Although this Patent Publication proposes a beam extraction method for extracting a charged-particle beam from an accelerator by applying a high-frequency electromagnetic field to the circulating beam to increase the amplitude of betatron oscillation, the Publication does not disclose any practical method of frequency control for radio frequency knockout (RF-KO).
Another example of a prior art approach is found in Japanese Examined Patent Publication No. 2833602 which discloses a charged-particle beam radiation system including a beam deflector, in which a charged-particle beam is extracted by using the beam extraction method of Japanese Examined Patent Publication No. 2596292. The beam deflector steers the beam to irradiate a desired spot with charged particles extracted by the aforementioned beam extraction method. Emission of charged particles is once stopped and resumed with the beam directed to a next spot of irradiation by the beam deflector by using the same extraction method. This process is repeated as many times as necessary.
A non-patent document titled “PROGRESS OF RF-KNOCKOUT EXTRACTION FOR ION THERAPY” published in the Proceedings of the European Particle Accelerator Conference (EPAC 2002), pp. 2739–2741, describes a technique for realizing high-speed beam extraction and cut-off operation with a uniform intensity of the extracted beam over time based on the beam extraction method of Japanese Examined Patent Publication No. 2596292.
Another non-patent document titled “Fast beam cut-off method in RF-knockout extraction for spot-scanning” published in Nuclear Instruments and Methods in Physics Research Section A, Volume 489 (2002), pp. 59–67, gives a more detailed description of the technique introduced in the aforementioned non-patent document titled “PROGRESS OF RF-KNOCKOUT EXTRACTION FOR ION THERAPY.”
Still another non-patent document titled “Advanced RF-KO slow-extraction method for the reduction of spill ripple” published in Nuclear Instruments and Methods in Physics Research Section A, Volume 492 (2002), pp. 253–263, provides a detailed description of a system control method.
According to the non-patent documents cited above describing a practical method of realizing the aforementioned charged-particle beam radiation system of Japanese Examined Patent Publication Nos. 2833602 and 2596292, three function generators are needed for generating high-frequency electric fields and it is necessary to control these three function generators as well as a high-frequency accelerator, in which transverse and longitudinal RF fields are turned on and off, for performing beam extraction and cut-off operation. This requires a complicated control system which results in an expensive beam radiation system, also causing a problem concerning equipment reliability which is most important for medical systems.
A synchrotron used in the charged-particle beam radiation system must radiate a charged-particle beam at varying energy levels and beam intensities. To radiate the charged-particle beam at a desired energy level and beam intensity, it is necessary to optimally control different beam parameters according to all possible conditions. Therefore, optimization of the parameters at construction and adjustment of the charged-particle beam radiation system is so time-consuming that the system becomes considerably costly.
The aforementioned non-patent documents propose arrangements employing a power supply for electromagnets having extremely high stability so that these arrangements do not cause any stability-related problem. If the stability of the power supply is lowered for the sake of cost reduction, however, resultant fluctuation in power supply voltage will cause limits of a stability region to fluctuate. Therefore, even if the charged-particle beam radiation system is entirely turned off, a beam will be emitted afterwards due to the power supply voltage fluctuation and this poses a serious problem.